1. Field of the Invention
The invention pertains to a method of crystallizing silicon, and more particularly, to a sequential lateral solidification (SLS) device and a method of crystallizing silicon using the same, in which alignment keys are formed on a substrate with one mask having a multiplicity of different patterns, and crystallization progresses in parallel to an imaginary line connecting the alignment keys with information for a distance between the mask and the alignment key.
2. Discussion of the Related Art
As information technologies develop, various displays increase in demand. Recently, many efforts have been made to research and develop various flat display panels such as a liquid crystal display device (LCD), a plasma display panel (PDP), an electroluminescent display (ELD), a vacuum fluorescent display (VFD), and the like. Some types of the flat display panels have already been used in various display devices.
LCDs are most widely used because of their beneficial characteristics and advantages including high quality images, lightweight, thin and compact size, and low power consumption. LCDs are therefore used as a substitute for cathode ray tubes (CRTs) for mobile image display devices. LCDs have also been developed for use in devices receiving and displaying broadcast signals, such as a television, a computer monitor, and the like.
An LCD device generally includes an LCD panel displaying an image and a driving unit for applying driving signals to the LCD panel. The LCD panel includes first and second glass substrates bonded to each other and a liquid crystal layer injected between the first and second substrates.
Multiple gate lines are formed on the first glass substrate (TFT array substrate) to be arranged in one direction at fixed intervals, and multiple data lines are arranged in perpendicular to the gate lines at fixed intervals. Multiple pixel electrodes are formed as a matrix in pixel regions defined by the gate and data lines crossing each other, and thin film transistors are switched by signals of the gate lines to transfer signals of the data lines to the pixel electrodes.
On the second glass substrate (color filter substrate), there is a black matrix layer for shielding light from other portions except the pixel regions, an R/G/B (Red/Green/Blue) color filter layer for realizing colors, and a common electrode for realizing an image.
Spacers maintain the above-described first and second glass substrates at a predetermined interval therebetween, and the substrates are bonded to each other by a sealant having a liquid crystal injection inlet. Liquid crystal is injected between the two glass substrates.
The driving principle of a general LCD device uses the optical anisotropy and polarization characteristics of liquid crystals. Because the structure of liquid crystal is thin and long, the liquid crystal molecules are aligned to have a specific direction. Based upon dipole moment, the liquid crystal molecules can have either positive or negative dielectric anisotropy. Applying an induced electric field to the liquid crystal controls the direction of the alignment. Therefore, when the alignment of the-liquid crystal molecules is arbitrarily controlled, the alignment of the liquid crystal molecules is eventually altered. Subsequently, due to the optical anisotropy of liquid crystals, light rays are refracted in the direction of the alignment of the liquid crystal molecules to thereby form images.
Recent technologies use an active matrix liquid crystal display (LCD), which is formed of a thin film transistor and pixel electrodes aligned in a matrix and connected to the thin film transistor. This technology is considered to have excellent high resolution and an ability to represent animated images.
In an LCD device having a polysilicon semiconductor layer for the thin film transistor, it is possible to form the thin film transistor and a driving circuit on the same substrate. Also, there is no requirement to connect the thin film transistor with the driving circuit, whereby the fabrication process is simplified. In addition, a field effect mobility of polysilicon is one to two hundred times higher than the field effect mobility of amorphous silicon, thereby obtaining a great stability to temperature and light.
The method of fabricating the polysilicon can be divided into a low temperature fabrication process and a high temperature fabrication process, depending upon the fabrication temperature.
The high temperature fabrication process requires a temperature condition of approximately 1,000° C., which is equal to or higher than the temperature for modifying substrates. Because glass substrates have poor heat-resistance, expensive and brittle quartz substrates having excellent heat-resistance should be used. When fabricating a polysilicon thin film by using the high temperature fabrication process, inadequate crystallization may occur due to high surface roughness and fine crystal grains, thereby resulting in poor device characteristics, as compared to the polysilicon formed by the low temperature fabrication process. Therefore, technologies for crystallizing amorphous silicon, which can be vapor-deposited at a low temperature, to form polysilicon have been researched and developed.
The method of depositing amorphous silicon at a low temperature and crystallizing the deposited amorphous silicon can be categorized into a laser annealing process and a metal induced crystallization process.
The laser annealing process includes irradiating a pulsed laser beam onto a substrate. More specifically, by using the pulsed laser beam, the solidification and condensation of the substrate can be repeated about every 10 to 100 nanoseconds. The low temperature fabrication process is acknowledged to have the advantage that the damage caused on a lower insulating substrate can be minimized.
The related art crystallization method of silicon using the laser annealing method will now be explained in detail.
FIG. 1 illustrates a graph showing the size of amorphous silicon particles versus laser energy density.
As shown in FIG. 1, the crystallization of the amorphous silicon can be divided into a first region, a second region, and a third region depending upon the intensity of the incident laser energy.
The first region of FIG. 1 is a partial melting region, where the intensity of the laser energy irradiated onto the amorphous silicon layer melts only the surface of the amorphous silicon layer. After irradiation, the surface of the amorphous silicon layer is partially melted in the first region, whereby small crystal grains form on the surface of the amorphous silicon layer after a solidification process.
The second region of FIG. 1 is a near-to-complete melting region, where the intensity of the laser energy, being higher than that of the first region, almost completely melts the amorphous silicon. After the almost complete melting, the remaining nuclei are used as seeds for a crystal growth, thereby forming crystal particles with an increased crystal growth as compared to the first region. However, the crystal particles formed in the second region are not uniform. The second region is also narrower than the first region.
The third region of FIG. 1 is a complete melting region, whereby laser energy with an increased intensity, as compared to that of the second region, is irradiated to completely melt the amorphous silicon layer. After the complete melting of the amorphous silicon layer, a solidification process is carried out, so as to allow a homogenous nucleation, thereby forming a crystal silicon layer formed of fine and uniform crystal particles.
In this method of fabricating polysilicon, the number of laser beam irradiations and a degree of overlap are controlled so as to form uniform large and rough crystal particles by using the energy density of the second region.
However, the interfaces between the multiple polysilicon crystal particles act as obstacles to the flow of electric current, thereby decreasing the reliability of the thin film transistor device. In addition, collision between electrons may occur within the multiple crystal particles can cause damage to an insulating layer due to the collision current and deterioration, thereby resulting in product degradation or defects. In order to resolve such problems, in the method for fabricating polysilicon by using a sequential lateral solidification (SLS) method, the crystal growth of the silicon crystal particle occurs at an interface between liquid silicon and solid silicon in a direction perpendicular to the interface. The SLS crystallizing method is disclosed in detail by Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, pp. 956-957, 1997.
In the related art SLS method, the amount of laser energy, the irradiation range of the laser beam, and the translation distance are controlled, so as to allow a lateral growth of the silicon crystal particle with a predetermined length, thereby crystallizing the amorphous silicon to a single crystal of 1 μm or more.
The irradiation device used in the related art SLS method concentrates the laser beam into a small and narrow region, and the amorphous silicon layer deposited on the substrate therefore cannot be completely changed into polycrystalline with a single irradiation. Therefore, in order to change the irradiation position on the substrate, the substrate having the amorphous silicon layer deposited thereon is mounted on a stage. Then, after irradiating a predetermined area, the substrate is moved so as to allow an irradiation to be performed on another area, thereby carrying out the irradiation process over the entire surface of the substrate.
FIG. 2 illustrates a schematic view of a related art sequential lateral solidification (SLS) device. Referring to FIG. 2, the related art sequential lateral solidification (SLS) device includes a laser beam generator 1, a focusing lens 2 focusing the laser beams discharged from the laser beam generator 1, a mask 3 to dividedly irradiate the laser beam on a substrate 10, and a reduction lens 4 formed below the mask 3 to reduce the laser beam passing through the mask 3 to a constant rate.
Generally, the laser beam generator 1 produces light with a wavelength of about 308 nanometers (nm) using XeCl or a wavelength of 248 nanometers (nm) using KrF in an excimer laser. The laser beam generator 1 discharges an unmodified laser beam. The discharged laser beam passes through an attenuator (not shown), in which the energy level is controlled. The laser beam then passes through the focusing lens 2.
The substrate 10 has an amorphous silicon layer deposited thereon, and the substrate 10 is fixed on an X-Y stage 5 that faces into the mask 3.
In order to crystallize the entire surface of the substrate 10, the X-Y stage 5 is minutely displaced, thereby gradually expanding the crystallized region.
FIG. 3 shows that the mask 3 includes an open part ‘A’ allowing the laser beam to pass through, and a closed part ‘B’ blocking the laser beam to prevent irradiation of the substrate. The width of the open part ‘A’ determines the lateral growth length of the grains formed after the first exposure.
FIG. 3 shows a plane view of a mask used in a laser irradiation process. FIG. 4 shows a crystallized region formed by a laser beam irradiation by using a mask of FIG. 3. Referring to FIG. 3, the mask used in the laser irradiation process is formed to have the open part ‘A’ having patterns opened at a first interval (a), and the closed part ‘B’ having patterns closed at a second interval (b). The open and closed parts alternate sequentially.
The laser irradiation process using the mask will be described as follows.
First, the mask 3 is placed over the substrate having an amorphous silicon layer deposited thereon, and then the first laser beam is irradiated onto the substrate. At this time, the irradiated laser beam passes through the multiple open parts ‘A’ of the mask 3, whereby predetermined portions 22 of the amorphous silicon layer corresponding to the open parts ‘A’ melts and liquefies, as shown in FIG. 4. In this case, the intensity of laser energy used herein has a value selected from the complete melting region, so that the silicon layer irradiated with the laser completely melts.
At this time, by one laser beam irradiation, the multiple open parts ‘A’ of the mask 3 correspond to one unit area 20 of the substrate, to which the laser beam irradiated, wherein the unit area 20 has a length ‘L’ and a width ‘S’.
After the laser beam irradiation, silicon grains 24a and 24b grow laterally from interfaces 21a and 21b between the amorphous silicon region and the completely melted and liquefied silicon region and towards the irradiation region. The lateral growth of the silicon grains 24a and 24b proceeds in a perpendicular direction to the interfaces 21a and 21b. 
In the predetermined portion 22 irradiated with laser for being corresponding to the open part ‘A’ of the mask, when the width of the predetermined portion 22 is narrower than the two times of the growth length of the silicon grain 24a, the grains growing inward in a perpendicular direction from both sides of the interface of the silicon region come into contact with one another at a grain boundary 25, thereby causing the crystal growth to stop.
Subsequently, in order to further grow the silicon grain, the stage bearing the substrate is moved to perform another irradiation process on an area adjacent to the first irradiated area. Thus, another crystal forms with the new crystal being connected to the crystal formed after the first exposure. Similarly, crystals laterally form on each side of the completely solidified regions. Generally, the crystal growth length produced by the laser irradiation process and connected to the adjacent irradiation part is determined by the width of open part ‘A’ and closed part ‘B’ of the mask.
FIG. 5 shows a grain boundary overlapped in each channel of devices formed along one line on a related art crystallization process.
In the related art crystallization process, the crystallization is performed on the entire surface of the substrate. Accordingly, when crystallizing the substrate, the stage bearing the substrate moves along one direction without any alignment key, to change the laser irradiation area onto the substrate.
However, as shown in FIG. 5, since the crystallization process proceeds without an index such as the alignment key, the grain boundary 25, which is the interface of the grains inside the laser irradiated region, is not rightly perpendicular to the length direction of the device (TFT, 30). Also, the number of grain boundaries overlapped in each channel of the respective devices is different. For example, one device may have one grain boundary 25, but another device may have two grain boundaries 25. That is, even though the respective devices are provided along one line, the number of grain boundaries 25 overlapped in each channel (between source electrode S and drain electrode D) of the respective devices may differ, and the respective devices will therefore have different mobility characteristics (as the number of grain boundaries 25 overlapped in the channel of the device becomes small, the mobility becomes rapid). Thus, the respective devices have the different characteristics even though the respective devices are provided along the same line.
FIG. 6 shows a plane view of respective areas formed on a substrate. Referring to FIG. 6, a thin film transistor array is formed on a substrate 50 of an LCD device. The substrate 50 is defined into a display area 70 of displaying an image, and a non-display area 60 around the display area 70.
At this time, an amorphous silicon layer is formed on an entire surface of the substrate 50. On the display area 70 of the substrate 50, multiple gate and data lines (not shown, formed on other portions except pixel regions of the display area) are formed perpendicular to each other, to define the pixel regions 71. Pixel electrodes are formed in the respective pixel regions 71. Also, a thin film transistor is formed at a predetermined portion of the pixel region 71, and the thin film transistor includes a gate electrode (not shown) protruding from the gate line, a source electrode (not shown) protruding from the data line, and a drain electrode (not shown) formed at a predetermined interval from the source electrode. At this time, a semiconductor layer 75 is formed below the source and drain electrodes, where the semiconductor layer 75 has a channel between the two electrodes. On the non-display area 60, respective driving circuit parts of gate and source drivers 61 and 62 are formed to provide signals to the gate and data lines.
However, the crystallization of the amorphous silicon layer formed on the substrate 50 requires different speeds in the respective areas. Especially, the driving circuit parts 61 and 62, and the thin film transistor (device, 75) of the display area 70 require a mobility speed that is one hundred times higher than those of the remaining portions. Accordingly, in order to decrease the process time and cost for the crystallization, one can omit the crystallization for the remaining portions which don't require the high mobility speed, and to perform the crystallization for the driving circuit parts 61 and 62 and the device 75 of the display area 70.
To crystallize the device 75 on the substrate of the display area 70, it is necessary to provide an index for laser irradiation areas of the substrate. For this, alignment keys are firstly formed on the substrate by a photolithographic process prior to crystallization. In this case, it is necessary to prepare additional masks for the alignment keys, thereby causing the increase of fabrication time and cost for preparation of the masks.
However, the related art SLS device and the method of crystallizing the silicon by using the same have the following disadvantages.
In the related art crystallization process, the crystallization is performed on the entire surface of the substrate without the additional alignment keys. Accordingly, when the substrate is loaded on the stage, the substrate may slide or turn aside. Alternately, when the mask is loaded to the mask stage, the crystallization process of the substrate progresses in a non-parallel fashion because the mask is not loaded at the correct position. However, there is no way to solve these problems in the related art.
The difficulties presented by the related art are especially acute when the gate line, the data line, and the thin film transistor (device) are formed by patterning after crystallization. In this case, the grain boundary formed by the related art crystallization process is not parallel or vertical to the line and device formed in the after-processing, but is instead slanted thereto. Accordingly, even though the respective devices are provided along one line, the respective devices have a different number of grain boundaries overlapped in the channel, which must be corrected since it may cause degradation of the display quality and reduced speed.
Meanwhile, since the crystallization of the amorphous silicon layer formed on the substrate requires the different speeds in the different respective areas, a selective crystallization process has been proposed to decrease the process time and cost for the crystallization. Here, to selectively crystallize the predetermined portion of the substrate, it is necessary to provide alignment keys to index the laser irradiation area on the substrate. In this case, the process requires both a mask for forming the alignment key and an additional mask for crystallization. Also, the process time is delayed due to the replacement time of the masks for alignment key formation process and crystallization process, and the cost of the process increases due to the masks.